Heretofore, 2-hydroxy-4-methylthiobutanoic acid has been produced by a process which comprises reacting 2-hydroxy-4-methylthiobutyronitrile with an equal molar or slight excess of sulfuric acid to hydrate and hydrolyze the nitrile compound, and separating the resulting reaction liquid into an aqueous layer and an oil layer containing 2-hydroxy-4-methylthiobutanoic acid. The aqueous layer, which contains ammonium bisulfate, is discarded without any after-treatment or discarded after neutralized with ammonia to form ammonium sulfate which is deposited and separated therefrom, see for example, U.S. Pat. No. 4,524,077 and U.S. Pat. No. 4,912,257. This process, however, is not an industrially recommendable process from the viewpoints of environmental friendliness as well as production cost, because it uses a large amount of sulfuric acid, forms a large amount of sulfates as by-products and moreover produces a large amount of sulfate-containing waste water.
A known process devised for reducing the amount of sulfuric acid used in the above-mentioned prior art process comprises thermally decomposing the by-produced sulfates to evolve SO.sub.3 and recovering the SO.sub.3 as sulfuric acid, see U.S. Pat. No. 5,498,790. However, this process requires complicated equipment for recovering sulfuric acid and requires a heavy investment for construction of the equipment.